This invention relates in general to electrophoresis and, in particular, to a capillary electrophoretic system for separating samples containing both positively and negatively charged components.
Capillary electrophoresis (CE) is emerging as one of the separation methods of choice in resolving a complex mixture into its constituents. In a typical capillary electrophoretic system, an electric field is applied across a capillary structure with typical dimensions of 2-200 microns inside diameter and 10-100 cm length. The medium is an electrolyte solution or a gel. In capillary zone electrophoresis (CZE), the capillary wall contains immobilized charges which cause the bulk solution as a whole to move under the influence of an electric field.
The above-described movement is known as electroosmotic flow. In CZE, each of the charged components of the sample also has electrophoretic mobility and components with different mobilities would migrate at different rates. Even though a sample may contain both positively and negatively charged components, since the electroosmotic flow rate is typically higher than the electrophoretic mobilities of most components, usually both positively charged and negatively charged components will be caused to move in the same direction when carried by the electroosmotic flow.
Where there is little or no electroosmotic flow in the capillary, it may be difficult in the conventional electrophoretic scheme to cause both positively charged and negatively charged components to migrate past the detector. For example, if a sample containing both negatively charged and positively charged components is injected into a gel-filled capillary at its cathodic end, the negatively charged components will migrate towards the anodic end of the capillary whereas the positively charged components will remain essentially at the cathodic end and fail to move. If the sample is injected at the anodic end of the gel-filled capillary, then the positively charged components of the sample will migrate towards the cathodic end and separate in the process while the negatively charged components will remain essentially at the anodic end and fail to move. The above is true also for separations using capillaries with treated inner walls to reduce or eliminate electroosmotic flow.
Since samples to be analyzed frequently do include both positively and negatively charged components, it is therefore desirable to provide a system that can be used to separate both positively charged and negatively charged components in a single run. None of the existing conventional electrophoretic systems is entirely satisfactory for solving this problem. It is therefore desirable to provide an improved electrophoretic system as a solution.